Anything not in spoiler text is, you guessed it, not a spoiler! Safe readings friends :)
Orange: Mirai, or Orange: Future, did exactly what you'd expect: give us a glimpse of the original future and the one where Kakeru lives. However, it splits its 60 minute run time between recaps and actual new content.
For a non-spoiler summary of it; the first 10 minutes are a recap of the show. Nothing new is added to this section at all. After that until the 35 minute mark, it goes over a different person's perspective on the recap. After 35 minutes is the new content. So, technically speaking, you're only really getting 30ish minutes of new stuff, unless if you count the 15 minutes of the new perspective.
For the new stuff that they did show, it adds a bit more to each character(even in the new perspective part) and gives us a clear idea of what happens after the events of the show, which is what a lot of people were looking for.
# Past this point, I'm going to talk more in depth about what they show in the new content. You have been warned!
Spoiler, click to view
A small thing is that they go over how the letters actually got delivered. They talk about it in the show itself, but they actually show them doing it in the movie.
Spoiler, click to view
Suwa talks about how he feels he and his future self are "selfish" for going after Naho when she was at a weak point in her life. Even future Suwa believes that if Kakeru was still alive, Naho definitely would've chosen him over Suwa. This is also shown by Naho absolutely rejecting and shattering Suwa at the shrine on New Years, saying that she only loves Kakeru. Poor guy.
Spoiler, click to view
They do go on to show that Kakeru winds up marrying Naho and having a child with her, which is exactly what Suwa expected(because Naho does only love Kakeru). Hagita and Azu also get together, which they announce to the group as they sat by the cherry blossom trees they all promised to visit. They also hang out a lot more and are generally a lot closer than what they are in the other future.
Spoiler, click to view
After they all watch the sunset over the town, Suwa from the first future wakes up and Naho tells him that she had the exact same "dream" as him, which leaves people wondering if this all truly was a parallel reality or if it was just a dream. Signs point to it being a parallel reality, however it's never fully explained. After they talk about the dream, it cuts to the beginning of the series where Naho's alarm doesn't go off. As her mom is trying to wake her, we see a letter addressed to Naho slip through the mail slot in the door before ending there.
# Do I think you should watch it?
As a person who loves filler content, I recommend it to all those who are looking for a little bit more. However, don't go into this expecting a whole lot more. There really isn't much there, just some things to confirm suspicions from the series. It was cute, continued the story just enough to satisfy me, and gave us more character interactions between the friends, both present and future. 
